Mechanical dysfunction of the low back is widely considered to be an important component of the large problem of idiopathic low back pain (LBP) and to involve abnormalities of vertebral motion (motion segment kinematics) and vertebral muscle use and/or morphology. Much has been learned concerning the end-points of the range of spinal motion but there is still limited understanding and treatment of these kinematic and muscle abnormalities. Recent investigations of these topics by the applicants and others have shown that: (1) the details of the motion between pairs of vertebrae are very finely controlled and repeatable in normal subjects (2) the expected linkage between abnormal motion and abnormal local muscle activity appears to be present, and (3) localized muscle morphometric changes occur in a number of cases of idiopathic LBP. Building on these studies, the proposed work has as its specific aims: 1) for each of the lumbar motion segments, to measure kinematic, EMG, and muscle size parameters during various range of motion and lifting tasks in normal subjects and in idiopathic LBP subjects that are considered likely to have abnormalities detectable by these measurements; 2) to test for correlation of these measurements with those of clinically available parameters; and 3) to test whether subjects can voluntarily alter their motion segment kinematic or EMG behaviors during task performance.